Depth Gauge Error and how it relates to the Cave Survey

[bgpartri]

I can probably help a bit by explaining how digital depth sensor works.

First, with most computers, the depth sensor is a digital, precalibrated device. That means that it transmits a binary number to the processor to indicate absolute pressure and temperature. The sensor itself is calibrated in the factory, and that calibration data is loaded from the depth sensor chip. There are some analog depth sensors out there, but not a lot.

The processor uses the factory calibration data and the reported temperature to adjust the absolute pressure reading. Then the computer designer decides what factor to use for water density. Water density varies across temperature and salinity. Computers may use fresh, salt, CE, or some other custom multiplier. If they use CE for example, that means that when the pressure varies by one atmosphere, the depth will vary by 10 meters.

Next comes altitude. To correctly determine depth with altitude compensation, some number must be used for the pressure at the air/water boundary. There are a variety of strategies to determine that pressure. Many computers have the user enter it. The computer could also capture the pressure when the computer is turned on.

Now, the calculations are the same, but the surface is no longer sea level, it is the derived air/water boundary pressure.

I don't understand your exact requirement, but I hope this helps.

Bruce

[rchrds]

Ok, I have tried to write this three times, and it is killing me. I hate the little response block.

Anyway, I think that most of the people here are missing the OP's point and question. The issue is not the vertical accuracy of the total survey in reference to a datum, but rather the accuracy of each shot based on the reporting of the depth by the depth gauge.

Using a pressure pot to calibrate the gauge is little better than taking two gauges to depth to check them against one another- unless you have a NIST certified pressure pot gauge or something.

Probably the real answer is as Forrest suggests- measure the absolute depth from the surface with a fiberglass tape and then create an error table for your gauge by comparing the reported depth with the actual depth from the tape. Of course, this is nominally useful- if your cave is shallower than the table you create with the tape, you are good to go- but if it is deeper- your accuracy is still in question below the table depth.

This is essentially the same thing you are doing with your Suuntos on a fixed course prior to surveying- you compare your instrument error with known values, and those values are entered into the book to be calculated by the number crunching dude along with the rest of your data later. It might be a little more involved, as you will probably have different error in different depth ranges- The error is rarely linear with depth gauges sensors, i.e. more error deeper, whereas a Suunto has a fixed error per instrument, regardless of what azimuth you are looking at.

But there is another problem with digital instruments that you dont have with Suuntos, and we are starting to see it with digital instruments like the DistoX. That is repeatability. Does the instrument consistently report the same solution every time? In the case of a depth gauge, if you went to the same cave, with the same surface water level every time, would it show the same depth at a fixed point every time? Maybe not. Unfortunately, there is no correction for this, and you are pretty much at the mercy of the hardware developer, and at what level of accuracy they (and you) wish to pay for a certain piece of electronic hardware.

But in the big picture, who cares? What kind of error are we really talking about anyway? And the answer is surveyors care, and this has been and continues to be a real issue for underwater cave surveyors who are interested in accuracy.

Let's look at the numbers.

Here is the new defacto standard for cave survey accuracy.

http://www.uisic.uis-speleo.org/UISmappingGrades.pdf

Unfortunately, right on the front page, the UIS folks decided that the accuracy standards do not apply to underwater survey. Why? Because our survey sucks so badly in comparison that they felt sorry for us and didn't want to have to downgrade every single underwater cave map marked Grade5 that was really a Grade 3 or less.

What's our problem? Lazy surveyors? Not hardly. The issue is instruments and time available. Look at most diving compasses. Can you READ a single degree? NO, you can interpolate a single degree, by estimating between the 3 or 5 degree markings. That's not the same at all. On a Suunto, you can read single degrees, and interpolate half or smaller fractions of a degree. That is accuracy. How about our distances? One can guess to the nearest foot, if you are using knotted line, most of the time, but some people suck pretty badly at that, particularly over long distances, suprisingly enough. You CAN use a tape, and get dry cave quality distance measurements, but it takes TIME. It can take twice as long to measure the same distance with tape than it does reading knots- and you have to have a good team dynamic to make it happen smoothly.

And finally, our original question, depth. The good old depth gauge. Plus or minus 1 foot. Let's use an X1 for example. Though the hardware specifications (it's been a while) read something like accuracy to 0.5 feet, that does us little good, as the gauge only displays the rounded foot value (no metrics here.) So, we could assume that it is accurate to the nearest foot, without the +/- one foot. But for other gauges, (VRxxx) that is not the case.

So, lets go back to the standards. What is required for a grade 5 map? According to the new UIS standard, (and the old, actually) the azimuth measurement must be to the nearest degree, the elevation (depth for us) the nearest degree, and distance, roughly 2 inches.

Right off, we are screwed for the azimuth. It takes a steady hand and good eye to get the nearest 2-3 degrees on most dive compasses, and still you are making an educated guess.

Distance, we can do it with a tape, but not with knotted line.

But how about elevation or depth? How much error is one foot, in the big picture, assuming you read it at the worst case? It depends on the length of your shot. If you are in the main passage at Hole in the Wall, and you have 100 foot shots, not very much. The equivalent elevation measurement is slightly more than 1/2 a degree- well within the grade 5 limits. But shorten the distance a bit- Even with the X1 or equivilant, assuming it is accurate to the nearest foot, at a 50 foot shot, we are over 1 degree elevation, not grade 5. At a normal shot length- say 12 feet- almost 5 degrees! That is Grade 3 work.

But WHOA! we are getting way out of hand here- There is one very important thing that is different between underwater cave survey and dry cave survey. In dry caves, the vertical control (distance below your datum, or some fixed point at the entrance) is ENTIRELY controlled by the accuracy of your vertical measurements- there is no other way, as you progress into the cave, to verify how much error you have accumulated, and thus you may think you are a hundred feet deeper than you actually are, because your clinometer was sticky and you measured wrong every time.

But think about underwater cave- when was the last time you thought you were 100 feet deeper than you actually were? (Snide comments aside.) The answer is you cant! Because your depth below the datum is not dependent on a series of connected depth measurements- you take an absolute measurement of depth in reference to your surface datum at every station. This means that the total vertical error of your survey is limited to the display error of your gauge at any given time- assuming you use the same water level and datum on every trip of the survey.

So, in closing, depth becomes a rather insignificant error in the big picture of your underwater cave survey- regardless of length of the survey, you are, if you treat your 0 datum correctly, limited to the error capability of your depth gauge- which if you consider somewhere like Peacock, with a depth potential of 180 some feet, a foot of vertical error over a range of 180 feet is less than one-half percent, Unheard of in the dry cave realm.

Note for the nitpickers: Of course, I am skipping the part that this whole discussion would have on individual shot foreshortening, thus errors in lateral loop closure. However, this is also a bit self limiting- the largest errors are on very short shots, which make less error in the loop, unless you have very small loops- sorry for ya.

Jason

[apitkin]

Jason,

That post is one of the most eloquent and well-constructed I have ever seen.

My only addition would be to consider the purpose of your survey. If you're exploring and interested in the possibility of new entrances, then experience has shown that the typical grade 3 (or worse) UW survey is usually enough. If you're trying to pinpoint cave passage from the surface (e.g. to drill a hole for water sampling, as in Manatee, or even a water-bottling plant), then it probably won't be. Most survey goes towards the construction of a map to aid other divers in navigating the cave, and even a good grade 1 sketch a la Hemphill can be adequate for that.

It is tempting to us all to try to achieve very high levels of accuracy underwater, but the effort required is not usually justified by the purpose of the survey. I'm not saying we should just do sloppy survey, just that we need to be realistic. Is a grade 3 survey of the whole cave more useful than a higher quality survey of just part of it?

Andy

[rchrds]

Andy, I would argue that at this time, greater than grade 3 accuracy is simply not possible under water. Primarily responsible for that is the inability to visually connect the stations with an accurate azimuth reading. Reading the line does NOT count- laying a compass alongside the line is simply not accurate, due to the short length of the compass edge and the non-linear nature of line in water, alignment is not accurate without sighting from one station to another visually. Length is well within our capabilities, and with a little electronics wizardry (and a lot of money), a depth gauge with tenths of a foot accuracy could be possible.

Jason

[jlillest]

Bruce, thanks for the details on how a digital pressure gauge works.

Jason, I knew I could count on you.

I guess the real answer is that the error in my depth gauge is not enough to worry about until I make my underwater topofil, and if I need to evaluate the accuracy of a survey I know a simple way to make a calibration table (Thanks Forrest!)

The underlying answer to my question is that if I want to get a good fix on the new entrance to the cave i dove this weekend, then I'm better off pumping out the sump and running a suunto & fiberglass tape survey through! I guess we'll start shopping for an adequate-sized pump....

Thanks all for the brain exercises, does anyone know if this topic has been published in any format in the past? I'm referring solely to depth guages (analog or digital) and the effect of their error on a survey? Compass & Tape (see SACS) is a publication that has worked out a lot of these issues for dry surveys and I'm pretty sure that Dasher's book has a good summary of everything you need to know to achieve high accuracy surveys.

-Jon

[rchrds]

No, a lot of this is sort of nit-pickery, and doesn't make for good reading an a book anyone would (or could) read. It might make for a good article in compass and tape, but I have been too lazy to join the section. There was, however, some VERY informative discussion at the last NSS convention from the folks up at Mammoth with regards to repeatability and accuracy of the DistoX, which provides some tangentially related information in regards to digital depth sensors. My largest wish this year is to find someone willing to take the DistoX azimuth chip and port it to a standard LCD output, so it can be mounted in a case and used underwater- We are all tired of peering at these completely ignorant analog compasses.

[rchrds]

Ok, just mailed $12 to SACS. (That was for three years, BTW) Now THAT is cheap dues.

[FW]

Ok, I have tried to write this three times, and it is killing me. I hate the little response block.

Click on "Reply to Thread", or compose long posts in Notepad.

BTW, nice post

[jason]

The issue is instruments and time available. Look at most diving compasses. Can you READ a single degree? NO, you can interpolate a single degree, by estimating between the 3 or 5 degree markings. That's not the same at all. On a Suunto, you can read single degrees, and interpolate half or smaller fractions of a degree. That is accuracy.

Jason

Ya'll may already be doing this - but a liquid-filled orienteering compass mounted to the top edge of a plastic survey slate will give you greater accuracy than a dive compass (1 to 2 degrees depending on the compass - mine is to two degrees). The addition of the survey slate increases the length of the 'compass' that is in contact with the line (you basically lay the entire contraption along the line).

[rchrds]

Ya'll may already be doing this - but a liquid-filled orienteering compass mounted to the top edge of a plastic survey slate will give you greater accuracy than a dive compass (1 to 2 degrees depending on the compass - mine is to two degrees). The addition of the survey slate increases the length of the 'compass' that is in contact with the line (you basically lay the entire contraption along the line).

That is what I am doing- compass mounted to my book (6 ring binder with plastic survey pages) but this is still a world away from 1 degree accuracy. Until you have tried to consistantly get matching front/backsights to one degree with suuntos, it is hard to get a grip on what exactly is required to make this happen, sufficit to say that no combination of currently use technology is going to do it. Though it may not always be visible to th naked eye, the line is ALWAYS curved, either hanging from its own weight, or from water flow, and this makes the possibility of a direct shot completely impossible.

Jason